The leaching of metals as soluble salts from various ores and techniques for separation of those salts from the pregnant liquid are well known. Many such leaching processes however require initial oxidation of the ore to oxidize the sulfides content of the ore and to release the metal salts. In other cases reduction of the salts in the slurry is required before the oxidation and subsequent leaching can occur.
Techniques for oxidation of the ore have long been appreciated as a problem area in the separation of the metals and most solutions have involved massive capital equipment and high inputs of energy for the purposes of heat and pressure oxidation.
For example Lakshmanan (in the 14th Annual CIM Hydro Metallurgical Meeting in Timmons, Ontario) disclosed techniques for roasting of refractory ores at high temperatures for purposes of oxidizing the sulfides and carbonaceous matter so that the resulting calcine residue is permanable and therefore amenable to cyanidation, that is the recovery of gold and silver by forming a cyanide complex.
The same reference discloses pressure oxidation which is used for refractory precious metal ores where a slurry of the concentrate is contacted with oxygen in an autoclave at elevated temperatures and pressures. Sulfides are oxidized to sulfur and/or sulfates, releasing the precious metals for cyanidation.
Another similar process disclosed is the Ontario Research Foundations Atmospheric Hydro Metallurgical Oxidation Process where "Caro's Acid" (Peroxymonosulphuric Acid H2SO5) is used to oxidize the refractory ore and then the oxidized pulp is further treated with cyanide to leach the gold and silver.
Chlorine Oxidation is used at the Carlin and Jeritt Canyon mills in Nevada to treat carbonaceous ores. The ore is slurried and then aerated at moderate temperatures in the presence of soda as to decompose soluble sulfides and some carbonaceous matter. This is followed by chlorine oxidation of the more refractory organic hydrocarbons. The resulting pulp is then treated with cyanide to recover the gold/silver.
Homestake Mining Company's McLaughlin gold project in Napa County, Calif. utilizes pressure oxidation of refractory sulfides and initially treated 2,700 tons-per-day of refractory gold ore. The process chemistry of the elevated temperature oxidation of the sulfides involves the formation of iron sulfate and sulfuric acid and the trivalent arsenic is oxidized to the ferric and arsenate states. Also, elemental sulfur is produced as well as jarosites. This technique is disclosed in a paper entitled Gold Extraction from Refractory Concentrates 14th Annual CIM Hydro Metallurgical Meeting, Timmons, Ontario by Robert Weir.
Peruvian mines utilize Pressure Oxidation of silver bearing sulfide flotation concentrates similar to the process used by Barrick at Mercury and Gold Strike mines. Problems encountered are 1) Excessive heat generation, 2) Silver solubility during autoclaving, and 3) Formation of pellets containing unreacted sulfides. This is disclosed in a paper entitled Pressure Oxidation of Silver Bearing Sulfide Flotation Concentrates SME Annual Meeting Pre-Print No. 92-59, by P. Thompson.
The United States Bureau of Mines holds a patent on a process for recovering nickel, iron and copper and gold and the platinum group metals from sulfide ore concentrates by the following:
1) Smelting with a flux to form a matte (copper-nickel sulfides).
2) Dry grinding of the matte (-200 mesh).
3) First Stage Leaching; Sulfuric Acid leaches the nickel and iron. Iron is removed by precipitation with ammonia after oxidation to ferric state. The nickel is removed by hydrogen reduction.
4) Second Stage Leaching; Ferric chloride, ferric sulfate, and cupric chloride is used. The copper is precipitated with H2S. The residue contains the precious/noble metals which are then processed.
Stillwater Mining Company in Montana operates an underground platinum-palladium mine and they make a sulfide concentrate by flotation and they then smelt the concentrates in a submerged-arc electric furnace and a top-blown rotary converter. The exhaust gases go directly to the scrubber and then cleaned in a dual-alkali wet scrubber to remove the sulfur dioxide before venting to the atmosphere.
Other processes for refractory gold ore are as follows:
1) Electrooxidation; producing sodium hypochlorite by electrolysis of brine to be used as an oxidant.
2) Hydrochloric acid-oxygenate leach of copper-nickel sulfide concentrates.
3) Ferrous chloride-oxygen leach of sulfide concentrates.
4) Microbial treatment of refractory ores with bioreductive bacteria.
5) Vacuum Liquidation Process can be used for selective recovery of tungsten-molybdenum-silver.
The limitations of the prior art technology are as follows:
1) Very high capital investment for pyrometallurgical treatment of concentrates.
2) High unit operating costs for pyrometallurgical treatment of concentrates.
3) Existing pyro-and-hydro metallurgical processes are inflexible and usually require a cyanide final leach.